text: In underwater diving the physiological effects of individual component gases of breathing gases are a function of partial pressure.
  Using diving terms, partial pressure is calculated as:
  :partial pressure = (total absolute pressure) × (volume fraction of gas component)
  For the component gas "i":
  :p = P × F
  For example, at  underwater, the total absolute pressure is  (i.e., 1 bar of atmospheric pressure + 5 bar of water pressure) and the partial pressures of the main components of air, oxygen 21% by volume and nitrogen approximately 79% by volume are:
  :pN = 6 bar × 0.79 = 4.7 bar absolute
  :pO = 6 bar × 0.21 = 1.3 bar absolute
  The minimum safe lower limit for the partial pressures of oxygen in a breathing gas mixture for diving is  absolute. Hypoxia and sudden unconsciousness can become a problem with an oxygen partial pressure of less than 0.16 bar absolute.   Oxygen toxicity, involving convulsions, becomes a problem when oxygen partial pressure is too high.  The NOAA Diving Manual recommends a maximum single exposure of 45 minutes at 1.6 bar absolute, of 120 minutes at 1.5 bar absolute, of 150 minutes at 1.4 bar absolute, of 180 minutes at 1.3 bar absolute and of 210 minutes at 1.2 bar absolute. Oxygen toxicity becomes a risk when these oxygen partial pressures and exposures are exceeded. The partial pressure of oxygen also determines the maximum operating depth of a gas mixture.
  Narcosis is a problem when breathing gases at high pressure. Typically, the maximum total partial pressure of narcotic gases used when planning for technical diving may be around 4.5 bar absolute, based on an equivalent narcotic depth of .
  The effect of a toxic contaminant such as carbon monoxide in breathing gas is also related to the partial pressure when breathed. A mixture which may be relatively safe at the surface could be dangerously toxic at the maximum depth of a dive, or a tolerable level of carbon dioxide in the breathing loop of a diving rebreather may become intolerable within seconds during descent when the partial pressure rapidly increases, and could lead to panic or incapacitation of the diver.